Electrophotographic reversal developing process

ABSTRACT

An electrophotographic reversal developing process where a charged electrophotographic element exposed to an optically negative image on an image bearing member where the area of the negative image is not over 10 percent of the area of the image bearing member. The electrophotographic element is next disposed with respect to a developing electrode which is completely electrically insulated from its surroundings to thereby induce on said electrode a potential which causes an electric field to occur at the image portions of the whole electrostatic latent image which is oppositely directed from that occurring at the background portion which corresponds to the area of maximum charge density. Next, toner particles having a charge of the same polarity as that of the background portion are introduced between the developing electrode and the electrophotographic element to develop the image portions.

United States Patent Tamai et al.

[54] ELECTROPHOTOGRAPHIC REVERSAL DEVELOPING PROCESS [72] Inventors: Yasuo Tamait Seiji Matsumoto; Satoru Honjo, all of Asaka-shi, Japan [73] Assignee: Fuji Photo Film Co., Ltd., Ashigara- Kamigun, Kanagawa, Japan [22] Filed: Nov. 12, 1969 -[2l] Appl.No.: 875,641

[30] Foreign Application Priority Data Nov. 12, 1968 Japan ..43/82696 [52] U.S.Cl. ..ll7/17.5,l17/37LE,118/637, 355/10 [51] Int.Cl. ..G03gl3/08,G03g13/10 [58] FieldofSearch ..1l7/l7.5,37LE;96/1;355/10 [56] References Cited UNITED STATES PATENTS 3,249,088 5/1966 Ostensen ..117/37 LE 3,391,082 7/1968 Maclay ..118/637 LE 3,336,906 8/1967 Michalchik ..l18/637 [15] 3,655,419 [451 Apr. 11, 1972 Lein... ..1 18/637 LE Uber ..ll7/17.5

[57] ABSTRACT An electrophotographic reversal developing process where a charged electrophotographic element exposed to an optically negative image on an image bearing member where the area of the negative image is not over 10 percent of the area of the image bearing member. The electrophotographic element is next disposed with respect to a developing electrode which is completely electrically insulated from its surroundings to thereby induce on said electrode a potential which causes an electric field to occur at the image portions of the whole electrostatic latent image which is oppositely directed from that occurring at the background portion which corresponds to the area of maximum charge density. Next, toner particles having a charge of the same polarity as that of the background portion are introduced between the developing electrode and the electrophotographic element to develop the image portions;

8 Claims, 7 Drawing Figures PATENTEDAPR H I972 655,419

SHEET 2 OF 2 Ill/I/I/f/I/I/I/ TNVENTOR5 ELECTROPHOTOGRAPHIC REVERSAL DEVELOPING PROCESS BACKGROUND OF THE INVENTION The present invention relates to electrophotography and more particularly to a so-called electrophotographic reversal developing process wherein a large amount of developpowder (toner particles) is deposited preferentially on areas having less electrostatic charge in an electrostatic latent image formed on an insulating layer and toner particles are not deposited on the areas thereof having the maximum electrostatic charge.

As is well know, when toner particles having an electrostatic charge of the same polarity as that of an electrostatic image formed on an insulator such as a photoconductive insulating layer are applied at the electrostatic latent image, the toner particles are repulsed by an electrostatic repulsive force at highly charged areas, while they deposit on discharged area. However, as in such a developing process the toner particles do not deposit on the insulating layer by an electrostatic attractive force, the deposited amount of the toner on the insulating layer corresponds not to the density of the electrostatic charges forming said electrostatic latent image but to the electrostatic field strength at the insulating layer surface. Accordingly, the toner particles deposit in a larger amount on the areas where the charge density charges discontinuously and hardly deposit on areas of uniform low charge density.

In order to realize toner deposition proportional to the discharged amount of charge, a process has been proposed wherein a direct current potential equal to the surface potential of the maximum charge density-bearing areas of the electrostatic latent image is applied between a developing electrode and the support of insulating material thus forming an electrostatic field therebetween which is most intense at the minimum charge density area and is weakest at the maximum charge density area, and accomplishing desired toner deposition.

Since the surface potential of the latent image-bearing surface of an insulating layer gradually decreases during development at darkness, the potential applied to the developing electrode is preferably decreased synchronized to the dark decay of the surface potential. However, the characteristics of the dark decay markedly depend upon the composition of insulating layer to be employed and surrounding conditions and hence are not so simple and it is quite difficult to vary the potential in accordance with the variation of the surface potential by the dark decay.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an electrophotographic reversal developing process overcoming the aforesaid difficulties.

The practical object of this invention is to provide a quite simple electrophotographic reversal developing process employing a developing electrode to which an outer potential source need not be connected, utilizing the electrostatic energy of the electrostatic latent image to be developed itself.

Other object of the present invention is to provide an electrophotographic reversal developing process by which a high density reproduction is easily obtained from an optically negative original such as micro film comprising light transmitting line image areas and dark background.

Still other object of the present invention is to provide an electrophotographic reversal developing process having a high developing speed.

According to the present invention, there is provided a reversal developing process of an electrostatic latent image formed on a photoconductive insulating layer on a conductive support using toner particles having an electrostatic charge of the same polarity as that of the electrostatic latent image to be developed wherein the photoconductive insulating layer is exposed to an optically negative image having an average transmittance of not over percent to the light of the spectral response range of said photoconductive layer to form an electrostatic latent image and then developing said latent image by placing an electrode adjacent the electrostatic latent image covering at least a part of the latent image-bearing area in such a manner that the electrode is electrically insulated from said conductive support and that the electrode is not connected with an outer power supply, thus the electric field at the space on the highlight portion in said latent image being substantially nullified, and supplying toner particles having the same polarity as that of the latent image to the space between said electrode and the latent image.

DETAILED DESCRIPTION OF THE DRAWING ment of an electrophotographic element used in the present invention,

FIG. 6 is a cross-sectional view showing another embodiment of an electrophotographic element used in this invention, and

FIG. 7 is a cross-sectional view showing an embodiment of the reversal developing process of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Now, in FIG. 1, an electrophotographic element composed of a conductive support 1 and a'photoconductive insulating layer 2 bearing thereon an electrostatic latent image is placed close to an electrode 3. In the embodiment the latent image is assumed to be formed by negative charges for convenience. That is, the latent image consists of negatively charged portions 4 and portions 5 having substantially no charges. In the present invention an optically negative image having a comparatively low average light transistance is utilized as an original and hence the area of the portion 4 is as a matter of course considerably larger than the area of the portion 5. In practice, a negative micro film having recorded thereon a line image with very low density against a background having a uniformly higher density is generally used in the present invention. In general, the area of the image in the negative micro film does not exceed 10 percent of the whole area of the film, the average image area being about 5 percent. Also since the difference in density between the image portion and the background portion is at least about 1.0, the ratio of the image area may be considered, generally speaking, to show directly the transmittance of the micro film.

When such an electrophotographic element having an electrostatic latent image corresponding to such optical image is allowed to approach the electrode or a conductor 3 which is electrically insulated from the support of the element with the recording layer faced to the electrode, the electrode 3 assumes a certain value of potential. That is, assume the ratio of the area of the latent image corresponding to the line image to the area of the whole latent image to be r, and the potential of said line image area to be 0, and the potential of the background portion to be v, then the electrode 3 will assume an average potential of (1-r)v induced by the charge on the background area.

Thus, if no other conductor having a noticeable capacity than said element is placed near the electrode 3, an electric field the same as that formed when a potential of rv is applied to the electrode 3 from an outer electric source will be formed between the electrode and the latent image-bearing layer. The direction of the electric field is opposite to that of the field by the latent image and the intensity of the field generally is multiplied by about l-r 1). Thus, as the sum of the both electric fields, an electric field having a definite intensity capable of attracting toner particles of negative polarity to the electrophotographic element is formed at the line image portion in the latent image 4 while at the background portion remains an electric field equals to (l-r)v, which is very weak when r is near 1, having opposite direction to that of the electric field at the image portion. As r cannot exceed 1, there is no trouble that an electric field of attracting the toner particles to the background portion is formed. In other words, according to the present invention, if it is desired to obtain a positive image from a negative image having a comparatively high average density by reversal development, the energy in the charged portion of the electrostatic latent image is effectively utilized without being wasted during the development. In this respect, the energy in the charged portion of a latent image is also utilized in a reproduction of line image by utilizing field inversion at image edges in a conventional reversal developing process but the process of the present invention wherein the energy in the charged portion is effectively utilized by the aid of a conductor or electrode has never been disclosed.

Thus, the advantages of the present invention as compared with a conventional reversal developing process utilizing no developing electrode are as follows:

1. An edge effect is less observed in the toner image formed finally, which becomes more apparent when a continuous tone image is included in a line image to be reproduced.

2. Since the intensity of the electric field at the line image portion is increased, the developing speed is increased, the effect of which is remarkable when an aerosol development or a liquid is employed.

Moreover, the developing process of the present invention has the following merits as compared with a conventional reversal developing process wherein a developing electrode applied with an outer electric potential is employed:

3. The developing apparatus is simplified and the process can be conducted by using a compact and inexpensive means.

4. It is unnecessary to adjust the electrode potential in accordance with the dark decay of the potential of latent image during development. In the present invention, the variation of the potential of the latent image caused by the dark decay is automatically compensated.

Such merits are particularly profitable in the case of employing a liquid development system wherein the potential of the photoconductive layer decays more rapidly than dry development systems, at the practice of the electrophotographic reversal developing process of this invention, the following points must be noticed:

a. Electrode 3 is electrically insulated.

This point must be particularly noticed when development is manually carried out. Also in the case of employing a conductive vessel for toner particles as the electrode, the vessel is carefully insulated from surroundings and the developing system is so constructed that the operator does not directly touch the electrode.

b. In particular, the electrode is carefully insulated from the conductive support of an electrophotographic element or the conductive part in the support.

As when the electrode is electrically connected to the conductive support, the reversal of electric field does not occur, this point must be particularly carefully noticed.

Furthermore, in the case of using as an electrophotographic element an Electrofax type paper employing a paper support, the paper support must be prevented from being directly brought into contact with the electrode when the paper support has a surface resistance of l0 l0Q/cm or less.

If a conductive intermediate layer such as a metallic layer or a conductive resin layer is formed on a paper support having a comparatively high electric resistance and an electrophotographic light-sensitive layer is formed on the intermediate layer, the conductive intermediate layer appears as a thin layer at the paper cross section or edge and it frequently occurs that the developing electrode contacts the thin layer at the cross section or edge of the paper. One effective attempt for overcoming this trouble is to apply an insulating resin compositionv or an insulating tape to the cross sections of the electrophotographic sheet or to provide an electrophotographic element as shown in FIG. 2 of the accompanying drawings wherein an intermediate layer having an area smaller than that of the element is inserted between the paper support and the electrophotographic light-sensitive layer so that the edges of the conductive intermediate layer do not appear at the cross sections. That is, in FIG. 2 is shown an embodiment of an electrophotographic element used in the present invention. The element is composed of a support 7 having a high electric resistance, such as a plastic film or a paper which has not been subjected to a conductive treatment, an electrophotographic light-sensitive layer 2, and a conductive intermediate layer 10 inserted therebetween and having an area smaller than that of the support and the electrophotographic light-sensitive layer. The conductive intermediate layer is, for instance, a metallic film or a resin layer having a high electric conductivity, such as, a polymer of monomers containing quaternary ammonium base or potassium polyvinyl-benzene sulfonate. Furthennore, a carbon black-containing layer may be employed as such an intermediate conductive layer.

By using the electrophotographic element as mentioned above, the developing electrode can be prevented from being shorted with the electrophotographic element.

In the practice of the present invention, a liquid developing system or a powder cloud developing system is most profitably employed as developing method.

Moreover, the invention was explained above by referring the case of using a negative original having line images for convenience but the reversal development of this invention can be effectively practiced by using any originals regardless of line image or continuous tone image if the average light transmittance of the original is less than 10 percent where the electrostatic energy in the charged portion in the latent image is always sufficient for causing the inversion of electric field.

Now, the following experiment will show that conventional electrophotographic reversal developments are inferior to the process of the present invention.

EXPERIMENT BY CONVENTIONAL REVERSAL DEVELOPMENT A mixture having the following composition was kneaded for 20 hours in a ball mill.

Photoconductive zinc oxide (SAZEX,

trade name of Sakai Chemical Co.) g. Vinyl chloride-vinyl acetate copolymer 36 g. Ethylphthalyl ethylglycolate 4 g. Toluene 35 ml. Butyl acetate 25 ml.

Immediately after kneading, the viscosity .of the white dispersion thus obtained was adjusted by adding further butyl acetate thereto and the resulting dispersion was applied to an aluminum layer of Metalmy Film (trade name, made by Toyo Rayon Co., Ltd.). Metalmy was an aluminized film prepared by vacuum depositing a thin aluminum layer on one surface of a polyethylene terephthalate film. The thickness of the photoconductive light-sensitive layer after drying was about 8 microns. After completely drying the layer, the electrophotographic element thus prepared was stored in the dark for 20 hours at 40 C. to be subjected to dark adaptation.

When the light-sensitive layer was subjected to corona discharge of -7,000 volts in subdued light of the layer accepted a surface potential of -450 volts. The light-sensitive layer thus charged was exposed for 2 seconds to a light source of 5,500 luxes having a color temperature of 2,000I(. through a transparent test chart shown in FIG. 3 of the accompanying and that of the area 12 was 0.12. The area of 12 was 8 percent of the whole area of the original.

After exposure, the light-sensitive layer was developed in a liquid developer containing negatively charged toner particles. The liquid developer was prepared by stirring a mixture of the following composition at a high speed.

Microlith Blue-M340 (trade name, made by CIBA, LTD.) 200 mg. A Varnish (made by Toyo Ink Mfg. Co.) 400 mg. Ethyl acetate 10 ml.

(*): Powders of a mixture of a blue pigment and a copolymer of 85 percent vinyl chloride and percent vinyl acetate.

(")z A varnish obtained by heating a mixture of rosin-modified phenol-formaldehyde resin and linseed oil. The blue pigment is said to be dispersed in the copolymer as substantially primary particles.

The blue and stable dispersion thus obtained was dispersed in a liquid having the following composition by means of an ultrasonic dispersing device;

Kerosene 250 ml. Cyclohexane 700 ml. Linseed Oil 50 ml.

By the above procedure, the copolymer was insolubilized and deposited together with or including the blue pigment to provide toner particles. The varnish dissolved in the liquid and contributed to disperse the toner particles thus deposited. As the toner particles dispersed in the liquid were quite fine, almost no turbidity was perceived in the blue liquid developer. That is, the particle size of the toner dispersed in the dispersion was less than 1 micron.

The liquid developer was placed in a stainless steel vat and the development of the aforesaid light-sensitive layer after exposure was carried out by immersing the light-sensitive layer in the liquid developer so that the latent image-bearing layer faced the bottom of the stainless steel vat. After conducting the development for 1 minute, the light-sensitive layer was rinsed with clear isoparaffin (Isopar-E, made by Esso Research & Development Co.) to remove the excessive liquid developer from the developed surface.

A number of light-sensitive layers as mentioned above were subjected to the aforesaid charging, exposing, and developing procedures and the results thereof were compared. If the development was satisfactory or correctly conducted wholeout the procedures, the portion of the light-sensitive layer corresponding to the portion 12 of the original should have been developed in a high density and the edge effect should have been scarcely observed at the portion.

However, in fact, the results showed that the development was conducted uneven. That is, in some case the development was conducted completely, whereas in other cases only the edge portion was developed in high density and the central portion of the image was developed in low density. By investigating the results of the difficulty, it was confirmed that the development was conducted imperfectly in the following cases:

i. In case where the aluminum layer of the support was electrically brought into contact with the stainless steel vat;

ii. In case where a metallic tweezers grasped by the operator was electrically brought into contact with the aluminum layer of the support and the stainless steel vat was earthed; and

iii. In case where an operators hand was brought into contact with the stainless steel vat, whereby the stainless steel vat was earthed.

The invention is explained more practically by referring to the following examples.

EXAMPLE 1 After charging the light-sensitive layer of the electrophotographic element as described in the aforesaid experiment, the four sides thereof were covered by an adhesive Mylar tape as shown in FIG. 4. The adhesive Mylar tape was composed of a polyethylene terephthalate film having coated on one surface an adhesive. The electrophotographic element used in the present invention was composed of a polyethylene terephthalate film 13 as a support for Metalmy, an aluminum layer 14, a light-sensitive layer 12, and the adhesive tape 15 covering the sides and edges of the element.

The light-sensitive layer thus insulated completely was developed in a stainless steel vat after being image exposed in a similar manner as in the aforesaid experiment. During the procedure, the stainless steel vat was carefully prevented from being earthed.

When the same procedure was applied to a number of lightsensitive layers, the development was conducted perfectly in all cases.

EXAMPLE 2 The almost same procedure as in Example 1 was followed with the exception that the sides of the photoconductive element were insulated by the adhesive Mylar tape before the light-sensitive layer was charged. In this case, since the charging of the light-sensitive layer became insufficient by the pressure of the adhesive tape, it was necessary to remove a part of the light-sensitive layer near the end of the sheet to expose the aluminum layer and to earth the exposed aluminum layer at charging. Before development, the exposed aluminum layer was insulated by an other piece of adhesive tape.

In this procedure, the development was conducted perfectly throughout the whole procedures.

EXAMPLE 3 The white zinc oxide dispersion described in the aforesaid a conductive treatment in a dry thickness of 8 microns.

Thereafter, the four sides of the light-sensitive paper were covered by an insulating resin as shown in FIG. 5, wherein a light-sensitive layer 2 was formed on a paper 1 subjected to conductive treatment and the sides of the element were covered by an insulating resin 16. Thus, the sides of the lightsensitive layer were completely insulated.

The light-sensitive layer was subjected to the charging, image exposing, and developing procedures as in Example I. The development was conducted in a stainless steel vat. During the development, the conductive support 1 was perfectly insulated from the stainless steel vat.

By the procedure mentioned above, the development was conducted perfectly and stable.

EXAMPLE 4 The edges of the photographic element as shown in FIG. 3 were bent upwardly as shown in FIG. 6, wherein the element was composed of a conductive support 1 and a light-sensitive layer 2 and the element was immersed in the liquid developer in a stainless steel vat so that the lower light-sensitive layer faced the bottom plate of the stainless steel vat.

With such configuration, the electric contact between the conductive support 1 and the bottom plate of the stainless steel vat was prevented and the development was conducted perfectly and constantly throughout the whole procedure.

EXAMPLE 5 The reversal development was conducted by using a vat shown in FIG. 7 of the accompanying drawing.

The vat 17 was composed of an insulating material and an electrode 18 embedded in the bottom of the vat. The area of the electrode 18 was smaller than that of the electrophotographic element and hence the electrode was effectively prevented from being electrically brought into contact with a conductive layer 14, such as an aluminum layer, of the electrophotographic element. The conductive layer 14 was placed between an insulating support 13 and a light-sensitive layer 2. The element was immersed in a liquid developer 19 having dispersed therein toner particles charged in the same polarity as that of an electrostatic latent image formed on the light-sensitive layer so that the light-sensitive layer faced the electrode 18 with a suitable interval therefrom.

By developing as above using the developing vessel, the development was conducted perfectly and constantly throughout the whole procedure.

What is claimed is:

1. An electrophotographic reversal developing process which comprises:

exposing an electrophotographic element comprising a uniformly charged photoconductive insulating layer on an electrically conductive support to an optically negative image on an image bearing member where the area of the negative image is not over percent of the area of said image bearing member to form on said photoconductive insulating layer an electrostatic latent image corresponding to said image;

positioning an electrically conductive development electrode with respect to at least a part of the latent imagebearing area where the developing electrode is electrically insulated from the aforesaid conductive support and where the developing electrode is electrically insulated from any source of potential voltage so that the electric field in the space adjacent to the region corresponding to the maximum charge density of said latent image is substantially nullified; and

developing said image by supplying toner particles having a charge of the same polarity as that of said latent image to the space between said electrode and the latent imagebearing layer.

2. The electrophotographic reversal developing process as set forth in claim 1 wherein said development is conducted in a liquid developer having toner particles dispersed in an insulating carrier liquid.

3. The electrophotographic reversal developing process as set forth in claim 2 wherein said liquid development occurs in a metallic developing vat where the bottom of said metallic vat is utilized as the developing electrode.

4. The electrophotographic reversal developing process as set forth in claim 2 wherein said liquid development occurs in a developing vat composed of an insulating material having said developing electrode disposed at the bottom of the vat.

5. The electrophotographic reversal developing process as set forth in claim 1 wherein said development step includes a powder cloud developing step.

6. The electrophotographic reversal developing process as set forth in claim 1 wherein said conductive support is insulated from the developing electrode by covering the four sides of the electrophotographic element including said conductive support with an insulating material.

7. The electrophotographic reversal developing process as set forth in claim 1 wherein said conductive support is insulated'from the developing electrode by inserting said conductive layer between an insulating support and the photoconductive insulating layer, the area of said conductive layer being smaller than that of the insulating support and the photoconductive layer.

8. The electrophotographic reversal developing process as set forth in claim 1 wherein the cross section or edge of the electrophotographic element is coated with an insulating material. 

2. The electrophotographic reversal developing process as set forth in claim 1 wherein said development is conducted in a liquid developer having toner particles dispersed in an insulating carrier liquid.
 3. The electrophotographic reversal developing process as set forth in claim 2 wherein said liquid development occurs in a metallic developing vat where the bottom of said metallic vat is utilized as the developing electrode.
 4. The electrophotographic reversal developing process as set forth in claim 2 wherein said liquid development occurs in a developing vat composed of an insulating material having said developing electrode disposed at the bottom of the vat.
 5. The electrophotographic reversal developing process as set forth in claim 1 wherein said development step includes a powder cloud developing step.
 6. The electrophotographic reversal developing process as set forth in claim 1 wherein said conductivE support is insulated from the developing electrode by covering the four sides of the electrophotographic element including said conductive support with an insulating material.
 7. The electrophotographic reversal developing process as set forth in claim 1 wherein said conductive support is insulated from the developing electrode by inserting said conductive layer between an insulating support and the photoconductive insulating layer, the area of said conductive layer being smaller than that of the insulating support and the photoconductive layer.
 8. The electrophotographic reversal developing process as set forth in claim 1 wherein the cross section or edge of the electrophotographic element is coated with an insulating material. 